Method for producing polylaurolactam powder compositions for the coating of metals at high temperatures

ABSTRACT

Polylaurolactam powder coating compositions for the coating of metals at high temperatures are obtained by grinding low molecular weight and brittle polylaurolactam with subsequent condensation by heating of the powder to temperatures of below its melting point. 
     A. for the production of pigment free polylaurolactam powders, the low molecular weight granules are ground at temperatures of between 0° and 45° C. which have a relative viscosity of between 1.25 and 1.45 (measured in a 0.5 percent m-cresol solution at 25° C.) and have been produced by the polymerization of laurolactam in the presence of 2 - 10 percent by weight of water, 0.3 - 0.7 percent by weight of phosphoric acid at temperatures of between 265° and 300° C. in the absence of chain stabilizers under internal pressure, with subsequent granulating, and extraction between 25° and 70° C. with methanol or ethanol. The subsequent condensation is conducted at temperatures of between 140° C. and below the melting point of the polyamide to relative viscosity value of between 1.55 and 1.65; and finally the powders obtained are brought to a particle size distribution of between greater than 60 and less than 350 microns. 
     B. for the production of pigmented polylaurolactam powders, the low molecular weight granules are treated at temperatures of between 110° and 140° C. and pressures of between 5 and 8 bars with the addition of pigments in primary aliphatic alcohols having 1 - 3 carbon atoms, cyclohexanol, or ethylbutanol, these granules having a relative viscosity of between 1.25 and 1.65 and likewise having been obtained by the polymerization of laurolactam in the presence of 2 - 10 percent by weight of water, 0.3 - 0.7 percent by weight of phosphoric acid at temperatures of between 265° and 300° C. in the absence of chain stabilizers under internal pressure, with subsequent granulating, and extraction between 25° and 70° C. with methanol or ethanol. The powders are obtained with gradual cooling to room temperature with a minimum of mechanical motion and these powders are dried, optionally screened and, if necessary, the thus-extracted granules or the dried powders are brought to relative viscosity value of between 1.55 and 1.65 by subsequent condensation at temperatures of between 140° C. and below the melting point of the polyamide.

CROSS REFERENCES TO RELATED APPLICATIONS

The disclosure of copending application Ser. No. 283,966, filed Aug. 28,1972 is incorporated herein.

Application Ser. No. 283,966 discloses a polylauryllactam and the stateof the art of producing polyamide powders both by pulverization andprecipitation from solution.

BACKGROUND OF THE INVENTION

The field of the present invention is polyamide powders for the coatingof metals.

The invention is particularly concerned with methods for the productionof polylaurolactam compositions for the coating of metals at hightemperatures obtained by grinding low molecular weight brittlepolylaurolactam with subsequent heating of the powder to temperatures ofbelow its melting point.

The state of the prior art of producing polyamide powders may beascertained by reference to U.S. Pat. Nos. 2,698,966; 2,742,440;2,975,128; 3,203,822; 3,299,009; 3,476,711; 3,927,141; and 3,966,838;British Pat. Nos. 535,138 and 830,757; West German Publishedapplications Nos. 1,570,392; 1,669,821; 1,210,183; 1,267,428; 1,520,551and 1,495,147; East German Pat. Nos. 7283; 23,607; 24,307 and 25,173;French Pat. Nos. 951,924 and 1,384,988, and Swiss Pat. No. 582,517; andthe Kirk-Othmer "Encyclopedia of Chemical Technology" 2nd Ed., Vol. 16(1968), under the section "Polyamide (Plastics)", pages 88-105,particularly page 92 - polylauryllactam (nylon-12), and polyundecanamide(nylon-11), page 101 Solution Processes, and Powder Processing, pages101-102, the disclosures of which are incorporated herein.

U.S. Pat. No. 2,698,966 discloses physical mixtures of different typesof nylon powders produced by dissolving the nylon in organic solventsand precipitating the powdered particles. In Example 14 of U.S. Pat. No.2,742,440, the solution of epsilon caprolactam in alcohol-water solutionand its precipitation as a powder is disclosed. The dispersion of Teflonpowder in a solution of epsilon caprolactam and the coprecipitation ofthe powders is disclosed in U.S. Pat. No. 2,975,128.

The flame spraying and fluidized bed coating of nylon on a metal base isdisclosed in U.S. Pat. No. 3,203,822. U.S. Pat. No. 3,299,009 disclosesthe n-methoxymethylation of nylons, and U.S. Pat. No. 3,410,832discloses the preparation of polymers and copolymers of lauryllactam.

It is known to use polyamide powder coating compositions for thepreparation of varnish-type coatings on metals. The coating operation isconducted in accordance with the fluidized bed coating method, the flamespraying method, or the electrostatic coating method. The polyamidepowders are obtained by precipitating the polyamide from solutions (textof German Patent application No. S 2805 39b 22/04, laid open on May 10,1951), or by grinding the polyamide granules, preferably at lowtemperatures under an inert gas atmosphere.

It is furthermore known to produce polyamide powders by grinding lowmolecular polyamides and then bringing the thus-obtained powders to thedesired relative viscosity or molecular weight by heating themconventionally to temperatures of below the melting point as disclosedin British Pat. No. 535,138; West German Published application No.1,570,392 and U.S. Pat. No. 3,476,711.

Polylaurolactam powders are also prepared in accordance with theseconventional methods and are used for coating purposes according toknown methods as disclosed in Chem. Ind., November 1968: 783-791, andModern Plastics, February 1966: 153-156. Since polylaurolactam powdersdo not always meet the required conditions of high elasticity,satisfactory edge coating, smooth surface, resistance to alkalineaqueous solutions, and in many cases tend especially to smoke duringprocessing, a large number of improvements have become known, such asplasticizer containing polylaurolactam powders, as disclosed in WestGerman Published application No. 1,669,821, those of a mixture ofhomopolylaurolactam and laurolactam-containing copolyamides, asdisclosed in U.S. Patent application No. 283,966, those containingpolyamides with N-alkoxymethyl groups in addition to acid reactingcatalysts, as disclosed in U.S. Pat. No. 3,966,838, or mixtures ofpolyamides having 8-11 aliphatically bound carbon atoms per carbonamidegroup, aminoplasts carrying alkoxyalkyl groups, and acid reactingcatalysts, as disclosed in U.S. Pat. No. 3,927,141. These powdersexhibit good properties in individual cases, but do not as yet fullysatisfy all of the required conditions.

SUMMARY OF THE INVENTION

Having in mind the limitations of the prior art, it has now beendiscovered that improved coating compositions of polylaurolactam, whichcan be applied in the form of a powder, are obtained, yielding coatingsat above their film-forming temperature without the production ofinterfering smoke with a smooth surface, a satisfactory edge covering,good elasticity, and excellent resistance against alkaline aqueoussolutions, by executing the following steps:

A. For the production of pigment free polylaurolactam powders, granulesare ground at temperatures of between 0° and 45° C. which have arelative viscosity of between 1.25 and 1.45 and have been produced bythe polymerization of laurolactam in the presence of 2-10 percent byweight of water, 0.3-0.7 percent by weight of phosphoric acid attemperatures of between 265° and 300° C. in the absence of any chainstabilizers under internal pressure, with subsequent granulating, andextraction between 25° and 70° C. with methanol or ethanol. A subsequentcondensation is then conducted at temperatures of between 140° C., andbelow the melting point of the polyamide to relative viscosity ofbetween 1.55 and 1.65; and finally the powders obtained are sized andbrought to a particle size distribution of between greater than 60 andless than 350 microns.

B. For the production of pigmented polylaurolactam powders, granules aretreated at temperatures of between 110° and 140° C. and pressures ofbetween 5 and 8 bars with the addition of the pigments in primaryaliphatic alcohols having 1-3 carbon atoms, cyclohexanol, orethylbutanol, these granules having a relative viscosity of between 1.25and 1.45 or 1.25 and 1.65 and likewise having been obtained by thepolymerization of laurolactam in the presence of 2-10 percent by weightof water, 0.3-0,7 percent by weight of phosphoric acid at temperaturesof between 265° and 300° C. in the absence of any chain stabilizersunder internal pressure, with subsequent granulating, and extractionbetween 25° and 70° C. with methanol or ethanol. The powders areobtained by precipitation with gradual cooling to room temperature witha minimum of mechanical motion. These powders are dried, optionallyscreened and, if necessary, the thus extracted granules or the driedpowders having a relative viscosity between 1.25 and 1.45 are brought toa relative viscosity of between 1.55 and 1.65 by subsequent condensationat temperatures of between 140° C. and below the melting point of thepolyamide.

The relative viscosities of the present invention, which are anindication of the molecular weight of the polylaurolactams of thepresent invention, are measured in a 0.5 percent meta-cresol solution at25° C. according to German Industrial Standard (DIN) 53,727.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The general methods of preparing polyamides in the presence ofphosphoric acids, useful in the present invention, are known where U.S.Pat. No. 2,557,808 discloses using concentrations between 0.01 and 10percent by weight; French Pat. No. 951,924 discloses, in the case oflaurolactam, less than 3 mol percent and approximately 1.5 percent byweight; East German Pat. No. 7,283 discloses 0.1 to 1 percent; SwissPat. No. 582,517 discloses 0.2 to 4 percent by weight; and West GermanPublished application No. 1,210,183 discloses 10⁻³ to 10⁻⁶ moles ofphosphorus per mole of lactam which corresponds to approximately 0.4 ×10⁻⁴ to 0.04 percent by weight.

Laurolactam is polymerized in the presence of phosphoric acid, usingtemperatures of between 300° and 330° C. and quantities of phosphoricacid of between preferably 0.1 and 0.3 percent by weight as disclosed inWest German Published applications Nos. 1,267,428 and 1,495,147, andDutch Published application No. 70/10033, the customary viscositystabilizers, such as mono- or dicarboxylic acids in particular, arefurthermore employed. It is known from East German Pat. Nos. 23,607,24,307, and 25,173 to use, when operating with phosphoric acid, minoramounts of water as well as minor amounts of phosphoric acid. From thismultitude of prior art, relating generally to the polymerizing art, noteaching can be derived with respect to optimizing the polymerization toobtain usable polylaurolactam powders satisfying all of therequirements, the more so since, as has been discovered, thepolymerization condition is merely one of the prerequisites to bemaintained in addition to the aforementioned, further conditions.

It is likewise known to extract prepolymers and then further to condensesame as disclosed in British Pat. No. 830,757, and U.S. Pat. No.3,476,711. This intermediate extraction is conducted in the productionof polylaurolactam powders to be employed for the aforementioned coatingpurposes within a narrow temperature range and only with strictlyselected extractants. It has furthermore been found that the powdersmust be prepared according to differing methods. Natural coloredpowders, which can contain stabilizers but must not contain anypigments, are produced according to the grinding method, wherein againstrictly selected temperature limits are maintained. The subsequentcondensation takes place according to conventional methods attemperatures of above 140° C. and below the melting point of thepolylaurolactam (179° C.), but however, the relative viscosity valuesmust lie in a narrow range. The particle size distribution of thepresent invention powder ranges between greater than 60 and less than350 microns, i.e. smaller or larger powder particles should not bepresent if at all possible. The distribution is such that 83 ± 5 percentis of a particle size of between 63 and 200 microns.

Pigment containing powders, however, are prepared according to theprecipitation method. In this mode of operation, the starting granulescan already exhibit the desired relative viscosity -(1.25 to 1.65), butit is advantageous from a manufacturing viewpoint to use the lowviscosity granules (1.25 to 1.45), since this is more economical for theproduction technique. The subsequent condensation is then effected afterthe extraction of the granules or also on the precipitated, driedpowders. Selected solvents are used in the precipitation process, and anarrow temperature range is maintained. The pigments are added duringthis precipitation stage. Accordingly, the mode of operation isdescribed by the following procedures.

POLYMERIZING STAGE

Laurolactam is heated under an inert gas atmosphere to 265°-300° C. inthe presence of 2-10 percent by weight of water and 0.3-0.7, especially0.4-0.5 percent by weight of phosphoric acid. The operation is carriedout under the thus-attained internal pressure ranging generally between14 and 25, preferably between 16 and 20 bars. The relative viscosityvalues range between 1.25 and 1.45, preferably between 1.33 and 1.43,particularly preferably between 1.35 and 1.40. The polymerization takesin general 3-8 hours. The polymer obtained is transferred at atemperature of 200°-260° C. and under a pressure of 9-15 bars, i.e. at asteam pressure of this magnitude, from the autoclave suitably into awater bath. The skein of polymer is comminuted, and the granulesproduced are extracted at 25°-70° C. with methanol or ethanol, which iscarried out simply by 2-3 times extraction by boiling with 2-3 times theamount by weight of the extractant. The dried granules are thereafterprocessed into powders.

(A) WITH THE USE OF A GRINDING PROCEDURE

The polylaurolactam must have a relative viscosity of between 1.25 and1.45, preferably 1.33-1.43, especially preferably between 1.35 and 1.40.The grinding step is executed in a mill as disclosed in U.S. Pat. No.3,476,711 at a temperature of between 0° and 45°, preferably between 10°and 35°, particularly preferably between 15° and 25° C. Only in thistemperature range is it possible to obtain round grains without cakingwhich additionally do not have an excessive amount of fines. When thegrinding step is carried out at temperatures of below 0° C., a highquantity of fines of below 60 microns is produced, so that the powdersare useless even after this proportion of fines has been separated.Besides, the fracture edges of the grain are sharper, and the capabilityof being fluidized becomes lower. When the temperature rises to valuesof above 45° C. during the grinding step, the grains exhibitsuperficially molten parts in the shape of spikes. The powders aresubjected to a subsequent condensation in the solid phase at 145°-165°,preferably 150°-160° C., to attain relative viscosity values of between1.50 and 1.65, preferably between 1.55 and 1.63. The desired sizing orremoval of the fines proportion less than 60 micron and of the coarseproportion greater than 350 microns takes place by sieve screening orair sifting.

(B) WITH THE USE OF A PRECIPITATION PROCEDURE

When pigmented polylaurolactam powders, or powders with additives whichcannot be introduced during the polycondensation are to be prepared,then the powders are produced by a precipitation process. For thispurpose, the polylaurolactam samples prepared as set out above andhaving relative viscosities of between 1.25 and 1.65, preferably between1.33 and 1.65, especially preferably between 1.40 and 1.60, are used.Suitable as precipitating agents are methanol, ethanol, propanol,2-ethylbutanol, and cyclohexanol. Ethanol is preferred, making itpossible to precipitate therein at about 135° C., and about 7 bars,50-60 percent of polylaurolactam. The additives, such as, for example,TiO₂ are added during the precipitating step and are thus incorporatedinto the powder. The type of precipitation is decisive for the granulardistribution of the powder obtained. With too vigorous an agitation andtoo rapid a cooling step, the results are powders having a finesproportion of 40-90 percent of less than 40 microns, unsuitable as apowder for fluidized bed treatment. In contrast thereto, when thecooling step is executed at about 1° C. per minute without agitation orvery slow agitation, only an extremely minor fines proportion of below40 microns is obtained. The powders are dried under an oil pump vacuumat 40°-100° C. and sized by screens or sifting as set forth above.

Suitable pigments used in modification (b) (precipitation procedure) aretitanium dioxide, carbon black, barium sulfate, iron oxide. They aregenerally added in amounts of about 1-20 percent by weight, preferably2-10 percent by weight, based on the polyamide. Pigments along theselines are also understood to include dyes, such as phthalocyanines andazo dyes. Antiaging substances and heat and light stabilizers,customarily present in an amount of 0.1-2 percent by weight, such assterically hindered phenols or triazines, can be added in case ofmodification (a) before, during, or after the polymerization and also,for example, during the grinding process or also during the subsequentcondensation step. The powders are useful especially in accordance withthe fluidized bed process, but can also be employed with the aid offlame-spraying or electrostatic coating methods. The layer thickness ofthe coating composition is in the case of the fluidized bed treatmentbetween 200 and 500 microns, and in the case of the flame-sprayingprocedure between 200 and 800 microns.

It is possible to use adhesion promoters.

The present invention is described in greater detail below with the useof specific examples. The relative viscosities were measured inmeta-cresol at a concentration of 0.5 g/100 ml. at 25° C. as disclosedin German Industrial Standard 53,727.

EXAMPLE 1

In an autoclave, 10 kg of laurolactam is heated for 7 hours to 280° C.together with 800 g. of water and 60 g. of 85 percent strengthphosphoric acid (corresponding to 0.51 percent phosphoric acid), thepressure thus attained is approximately 19 bars. Thereafter, the mixtureis cooled to 240°-230° C. (the pressure being about 13-14 bars) and thebottom valve is opened. The polymer is conducted into a water bath by awater jet impinging on the skein of polymer having an angle of about 90°and, after cooling, is comminuted or ground in an impeller mill. Thepolylaurolactam is dried under a pressure of about 30 mbar and at atemperature of 60°-80° C. The relative viscosity is 1.39. The extract,determined by two hours of boiling of a sample with twice the amount ofmethanol and concentration of the extract to dryness, amounts to 1.5percent by weight.

The polymer particles are refluxed for 2 hours with twice the amount byweight of methanol. The methanol is discharged and replaced by the sameamount of fresh methanol. The mixture is once more refluxed for twohours, the methanol is discharged, and the mixture is dried at 30 mbarand 80° C. The polylaurolactam is ground in an impact pulverizer at acycle gas temperature of 20° C. The pulverizer is maintained at atemperature of 20° C. by introducing liquid nitrogen in increments. Thepowder has a fines proportion of less than 5 percent of below 60microns. Thereafter, the powder is subjected to a subsequentcondensation in a drying chamber in a thin layer by passing nitrogenthereover at a temperature of 145° C. for a period of 5 hours. Thepowder then has a relative viscosity of 1.59.

The coating of metallic components in accordance with the fluidized bedcoating step with the use of the polylaurolactam powder yields thefollowing evaluation: no smoking during the coating step, goodfluidizability, smooth coating surface, good to very good covering ofsharp edges, Erichsen depression greater than 10 mm, impact depressiongreater than 1800 mm/7.6 kg., resistance with respect to 1 percentboiling alkaline solution of a coarse detergent greater than 2000 hours.

EXAMPLE 2

The polylaurolactam ground and extracted in accordance with Example 1 issubjected to a subsequent condensation in a forced circulationevaporator by passing nitrogen over the material for 4 hours at 150° C.The polymer then has a relative viscosity of 1.60.

A glass autoclave is charged with 100 g. of the polylaurolactam, 200 ml.of ethanol, and 7 g. of TiO₂ pigment. The mixture is heated for 2 hoursto 140° C., thus attaining a pressure of about 7 bars. Thereafter, acooling step at 1° C. per minute is conducted at an agitating speed ofabout 5 r.p.m. The precipitated powder is dried in a drying chamber at0.7 mbar and 80° C. In the coating of metallic components, thewhite-dyed powder receives in all items the same satisfactory evaluationas in case of the natural colored powder of Example 1.

EXAMPLE 3

The polylaurolactam ground and extracted in accordance with Example 1 issubjected to a subsequent condensation in a forced circulationevaporator by passing nitrogen over the material for 4 hours at 150° C.The polymer then has a relative viscosity of 1.60. 100 g. of thepolylaurolactam is refluxed for 2 hours together with 7 g. of TiO₂pigment and 300 ml. of 2-ethylbutanol and then cooled under slowagitation. The powder is dried at 0.7 mbar and 80° C. The finesproportion of less than 60 microns is screened. The thus produced powderpossesses the same coating properties as the powders described above.

COMPARATIVE EXAMPLE 1

400 g. of granulated polylaurolactam, obtained by the hydrolyticpolymerization of laurolactam in the presence of 3 percent by weight ofwater and 0.2 percent by weight of adipic acid, wherein in the firststage a polycondensation is conducted at an internal pressure of between16 and 20 bars at 280° C., and after removal of the water and afterexpansion, a subsequent condensation step is executed at 260° C., inaccordance with West German Published Application 2,152,194, relativeviscosity = 1.60, is heated to the boiling point for 5 hours with 30 g.of TiO₂ pigment in 900 ml. of dimethylformamide. After cooling, thepowder is dried at 0.7 mbar and 80° C., and the fines proportion of lessthan 60 microns is removed by screening.

The powder shows the following grading in the coating process:

Good capability for fluidizing, strong smoking during the coatingoperation, insufficient covering of sharp edges, Erichsen depression 6-8mm., impact depression less than 100 mm/7.6 kg., resistance to 1 percentboiling alkaline solution of a coarse detergent less than 200 hours.

COMPARATIVE EXAMPLE 2

In accordance with the process described in Example 1, a polylaurolactamis produced without the addition of phosphoric acid (relative viscosity= 1.42) and, as described in Example 1, extracted, ground, andsubsequently condensed in the solid phase to relative viscosity value of1.60. The fines proportion of less than 60 microns is removed byscreening.

The powder shows the following coating characteristics: no smokingduring the coating step, no coating on sharp edges, Erichsen depression6-8 mm., impact depression less than 100 mm/7.6 kg., resistance to aboiling washing solution of a coarse detergent less than 250 hours.

COMPARATIVE EXAMPLE 3

300 g. of the polylaurolactam described in Example 2 and used for theprecipitation step, having a relative viscosity of 1.60, is heated toboiling for 5 hours together with 21 g. of TiO₂ in 800 ml. ofdimethylformamide. After cooling, the powder is dried at 0.7 mbar and80° C. The fines proportion of less than 60 microns is removed byscreening.

Coating properties: no smoking during the coating step, no covering ofthe coating on sharp edges, Erichsen depression 5 mm., impact depressionless than 100 mm/7.6 kg., resistance to a boiling alkaline solution of acoarse detergent less than 200 hours.

COMPARATIVE EXAMPLE 4

Comparative Example 3 is repeated with the difference that2-ethylhexanol is used in place of dimethylformamide for precipitationpurposes. During the coating step, the same evaluation is obtained asset forth in Comparative Example 3.

COMPARATIVE EXAMPLE 5

In accordance with the method described in Example 1, a polylaurolactambatch with a phosphoric acid addition of 0.1 percent by weight andanother with a phosphoric acid addition of 1 percent by weight areprepared (relative viscosities = 1.43 and 1.40, respectively) andextracted, ground, and subsequently condensed to a relative viscosity of1.61 and 1.59, respectively, in the solid phase, as in Example 1. Thepowders show the following coating properties:

    ______________________________________                                        (a) 0.1% H.sub.3 PO.sub.4                                                                      (b) 1% H.sub.3 PO.sub.4                                      ______________________________________                                        no smoking       slight smoking                                               satisfactory coating on                                                                        no coat on sharp edges                                       sharp edges                                                                   Erichsen depression                                                                            Erichsen depression <8 mm.                                   <10 mm.                                                                       impact depression <100                                                                         impact depression <200                                       mm/7.6 kg.       mm/7.6 kg.                                                   resistance with respect to                                                                     resistance with respect to boiling                           boiling 1% alkaline solution                                                                   boiling 1% alkaline solution                                 <500 hours       <300 hours                                                   ______________________________________                                    

COMPARATIVE EXAMPLE 6

Polylaurolactam powders are produced in accordance with Example 1,except that the grinding temperature is 50° C. in the first test and-10° C. in the second test. The powders are subsequently condensed to arelative viscosity of 1.59 in analogy to Example 1.

(A) TEST 1

The fluidizing capability of the powder is so poor that no usefulcoating can be produced by following the fluidized bed process.Observation under a microscope reveals that the grains have spike-shapedcakings.

(B) TEST 2

The fines proportion of the powder of less than 60 microns is so largethat a removal by screening becomes technically unreasonable. The powderitself is fluidized only with great difficulties and is unsuitable forfluidized bed coating.

The following Table comprises the results of the tests from the examplesand comparative examples. It can be seen that only if all necessarymeasures, as described, are being combined, it is possible to obtainpolylauorlactam powders having good properties in all testing methods.

                  TABLE                                                           ______________________________________                                                               Erich-  Impact                                                                sen     De-                                                                   De-     pres-                                                                 pres-   sion   Alkali                                         Smok- Edge      sion    mm./   Test                                           ing   Coat      mm.     7.6 kg.                                                                              Hours                                   ______________________________________                                        Example 1                                                                              No      Very Good >10   >1800  >2000                                 Example 2                                                                              No      Very Good >10   >1800  >2000                                 Comparative                                                                   Example 1                                                                              Strong  Insufficient                                                                            6-8   <100   <200                                  Comparative      Practically                                                  Example 2                                                                              No      Uncoated  6-8   <100   <250                                  Comparative      Practically                                                  Example 3                                                                              No      Uncoated  5     <100   <200                                  Comparative      Practically                                                  Example 4                                                                              No      Uncoated  5     <100   <200                                  Comparative                                                                   Example 5 (a)                                                                          No      Good      >10   <100   <500                                  5 (b)    Minor   Practically                                                                             <8    <200   <300                                                   Uncoated                                                     Comparative                                                                   Example 6 (a)                                                                          Unus-                                                                         able                                                                 6 (b)    Unus-                                                                         able                                                                 ______________________________________                                    

We claim:
 1. A method of producing pigmented polylaurolactam powderscomprising:(a) polymerizing laurolactam in the presence of about 2 to 10percent by weight of water, about 0.3 to 0.7 percent by weight ofphosphoric acid, and at a temperature of about 265° to 300° C., in theabsence of chain stabilizers under an internal pressure of about 14 to25 bars to a polylaurolactam having a relative viscosity of about 1.25to 1.65; (b) granulating said polylaurolactam of (a); (c) extractingsaid granulated polylaurolactam with ethanol or methanol at atemperature of about 25° to 70° C.; (d) mixing said extractedpolylaurolactam with about 1 to 20 percent by weight pigment dispersedin a liquid selected from the group consisting of primary aliphaticalcohols having 1-3 carbon atoms, cyclohexanol and ethylbutanol andtreating the mixture at a temperature of about 110° to 140° C. andpressures of about 5 to 8 bars; and (e) cooling the treated mixture of(d) to room temperature with precipitation of said pigmentedpolylaurolactam powders.
 2. The method of claim 1, wherein saidpigmented polylaurolactam powders are sized to a particle distributionof between greater than about 60 and less than about 350 microns.
 3. Themethod of claim 1, wherein cooling step (e) is carried out at the rateof 1° C. per minute.
 4. The method of claim 1, wherein said relativeviscosity is 1.33 to 1.65.
 5. The method of claim 1, wherein saidrelative viscosity is 1.40 to 1.60.
 6. The method of claim 1, whereinsaid pigment is selected from the group consisting of titanium dioxide,carbon black, barium sulfate and iron oxide.
 7. A method of producingpigmented polylaurolactam powders comprising:(a) polymerizinglaurolactam in the presence of about 2 to 10 percent by weight of water,about 0.3 to 0.7 percent by weight of phosphoric acid and at atemperature of about 265° to 300° C. in the absence of chain stabilizersunder an internal pressure of about 14 to 25 bars to a polylaurolactamhaving a relative viscosity of about 1.25 to 1.45; (b) granulating saidpolylaurolactam of (a); (c) extracting said granulated polylaurolactamwith ethanol or methanol at a temperature of about 25° to 70° C.; (d)mixing said extracted polylaurolactam with about 1 to 20 percent byweight pigment dispersed in a liquid selected from the group consistingof primary aliphatic alcohols having 1-3 carbon atoms, cyclohexanol andethylbutanol and treating the mixture at a temperature of about 110° to140° C. and pressures of about 5 to 8 bars; (e) cooling the treatedmixture of (d) to room temperature with precipitation of pigmentedpolylaurolactam powder; and (f) condensing the precipitated powder of(e) at a temperature between about 140° C. and below the melting pointof said polylaurolactam to said pigmented polylaurolactam powders havinga relative viscosity of about 1.55 to 1.65.
 8. The method of claim 7,wherein said pigmented polylaurolactam powders are sized to a particledistribution of between greater than about 60 and less than about 350microns.
 9. The method of claim 7, wherein cooling step (e) is carriedout at the rate of 1° C. per minute.
 10. The method of claim 7, whereinsaid pigment is selected from the group consisting of titanium dioxide,carbon black, barium sulfate and iron oxide.